Charge pump with current limiting circuit

ABSTRACT

A charge pump power supply includes two or more modes of operation. An input protection circuit is connected between an input of the power supply and a voltage source. The input protection circuit regulates the voltage at the input of the power supply, limits current at the input when switching from a weaker mode to a stronger mode, and prevents current reversal when switching from a stronger mode to a weaker mode. In some modes, the power supply continuously provides current to the load, obviating the need for an output capacitor.

FIELD OF THE INVENTION

[0001] The present invention relates to switching power supplies, andmore particularly, to a charge pump switching power supply with voltageregulation at its input.

BACKGROUND OF THE INVENTION

[0002] Portable electronic devices are ubiquitous, and are increasing inboth number and diversity. Examples include cellular telephones,personal digital assistants (PDAs), portable computers, portabletape/CD/MP3 players, hand-held televisions, and the like. These portabledevices are generally powered by a battery, which may be rechargeable,such as nickel cadmium, nickel metal hydride, or lithium ion, or whichmay be of a single-use type, such as conventional alkaline cells. All ofthese batteries lose their charge over time, and consequently do notproduce a single, constant voltage output.

[0003] To simplify the design of the electronics operating on batterypower, the time-varying output voltage of the battery must bestabilized, or regulated, to a constant, predetermined value. Inparticular, as the battery charge decays, its output voltage may fallbelow the required operating voltage of the electronics, necessitating a“boost,” or increase of the battery's voltage. One way to boost an inputvoltage, such as that from a battery, to a higher output voltage is byuse of a type of switching power supply known generally in the art as acharge pump. A charge pump typically contains one or more charge storagecapacitors, also known in the art as bucket capacitors or boostcapacitors, whose interconnection configuration relative to the inputand output nodes and a circuit ground is configurable via a network ofswitches.

[0004] A charge pump is often preferable to a linear voltage regulatoras a power source for electronics due to its efficiency, and its abilityto boost voltages from input to output. A charge pump may be preferredover a switched mode power supply based on reactive elements due to itsease of use, relatively low noise, and lower cost. Factors that haveprevented the widespread use of traditional charge pumps as previouslyknown in the art include poor efficiency and limited output loadcurrent.

SUMMARY OF THE INVENTION

[0005] The present invention relates to a type of switching power supplygenerally known in the art as a charge pump. The switching power supplyis connected in series between a power source, such as a battery, and aload to provide a generally constant output voltage to the load. Theswitching power supply comprises a switching circuit, a control circuit,and an input protection circuit. The switching circuit comprises anetwork of switches and charge storage capacitors, also known in the artas bucket capacitors. The switches of the switching circuit areselectively actuated under the direction of the control circuit tocharge and discharge capacitors within the switching circuit to providea regulated voltage at the output of the switching circuit. Theregulated output voltage is fed back to the control circuit, whichcontrols the operation of the switching circuit. The control circuitcompares the output voltage with a reference voltage and generates asignal that controls the operation of the switching circuit. The inputcircuit has a plurality of modes that provide different ratios of inputto output voltage.

[0006] An input circuit is connected in series between the power sourceand the input of the switching circuit. The input circuit serves threefunctions: 1) to prevent voltage breakdown of the switches in theswitching circuit; 2) limiting currents from the power supply duringswitching transitions; and 3) preventing current reversal through theinput of the switching circuit during switching transitions.

BREIF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram depicting the charge pump in a typicalcircuit.

[0008]FIG. 2 is a circuit schematic depicting one embodiment of aswitching circuit according to the present invention.

[0009]FIG. 3a depicts circuit schematics of an improved 1.5× mode of acharge pump, in two phases.

[0010]FIG. 3b depicts circuit schematics of a 2× charge pump, in twophases.

[0011]FIG. 4 is a block diagram of the control circuit for a chargepump.

[0012]FIG. 5 is a graph of the efficiency of a multi-mode charge pumpswitching power supply over a range of battery voltages.

[0013]FIG. 6 depicts an input voltage regulator, with current limitingand reverse bias input circuits.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention relates to a type of switching power supplygenerally referred to in the art as a charge pump or switched capacitorcircuit. The switching power supply is connected in series between apower source 26, such as a battery, and a load 24 to provide a generallyconstant output voltage to the load. A block diagram of an exemplaryswitching power supply, denoted generally by the numeral 10 is shown inFIG. 1.

[0015] The switching power supply 10 comprises a switching circuit 12,control circuit 14, and input circuit 16. The function of the switchingcircuit 12 is to provide an output voltage, denoted V_(OUT), at anoutput of the switching circuit 12 that is derived from a regulatedinput voltage, denoted V_(IN), that varies between selected voltagelevels. Switching circuit 12 comprises a network of switches and chargestorage capacitors, also known in the art as bucket capacitors or boostcapacitors. The switches of the switching circuit 12 are selectivelyactuated under the direction of the control circuit 14 to charge anddischarge capacitors within the switching circuit to provide a regulatedvoltage at the output of the switching circuit. The regulated outputvoltage V_(OUT) is fed back to the control circuit 14, which controlsthe operation of the switching circuit 12. Control circuit 14 comparesthe output voltage V_(OUT) with a voltage reference and generates apulse train that switches the switching circuit 12 between phases. Inputcircuit 16 is connected in series between the power source 26 and theswitching circuit 12.

[0016]FIG. 2 depicts one exemplary embodiment of the switching circuit12. The switching circuit 12 comprises two charge storage capacitors C1and C2 and a plurality of switches S1-S10. The switching circuit 12depicted in FIG. 2 may in general be implemented in a wide variety ofways. In one embodiment particularly suited to portable electronicsapplications, the switching network comprising switches S1-S10 areimplemented on an integrated circuit, with the various switchescomprising transistors, as well known in the art. Charge storagecapacitors C1 and C2 are mounted off chip, as is output capacitor 22,also referred to herein as C_(OUT).

[0017] The various switches S1-S10 of the switching circuit 12 may beselectively actuated to configure charge storage capacitors C1 and C2 ina plurality of different configurations or circuit topologies withrespect to the voltage input and output nodes V_(IN) and V_(OUT),respectively, and with respect to the circuit ground. For purposes ofthis application, the various configurations of the switching circuit 12will be described by reference to modes and phases. The term mode refersto a set or group of configurations that collectively produce a fixedgain or ratio of output to input voltage. For example, the switchingcircuit of the exemplary embodiment comprises a 1× mode, 1.5× mode, a 2×mode, and a 3× mode. During each mode (with the exception of the 1×mode), the capacitors C1 and C2 may be switched between two or moreconfigurations in which they are alternately charged by the inputvoltage and discharged to the output. In this way, electrical charge is“pumped” from the input node to the output node. These repetitiveswitched configurations within a mode are referred to as “phases” of thegiven mode of operation. Thus, as used herein, the term mode refers to aspecific repetitive sequence or cycle of phases that cooperativelyfunction to produce a regulated output voltage at a unique gain or ratioof output voltage to input voltage. The term phase refers to the circuitconfigurations that are the constituent cycles or elements of a mode.Each gain mode comprises at least two phases (the 1× mode comprises onlyone phase), and a given implementation of switching circuit 12 maycomprise more than one mode.

[0018]FIGS. 3A and 3B illustrate in simplified form some of the modesand phases of the switching circuit 12. The state of switches S1-S10 ineach of the modes and phases is shown in Table 1 below: TABLE 1 SwitchConfigurations (closed switches) Mode Phase 3x 2x 1.5x 1x Charge S1, S2,S6, S9 S1, S2, S6, S9 S1, S2, S6, S9 S4, S9, S2, S10, S6, S8 (Phase A)Pump S3, S5, S8, S3, S4, S8, S10 S4, S7, S10 S4, S9, S2, S10, S6, S8(Phase B) (same as Phase A)

[0019] In Table 1, the columns represent various modes and the rowsrepresent distinct phases (with the exception of 1× mode). The switchesdesignated in Table 1 are those that are closed during the designatedphase.

[0020]FIG. 3A depicts the phases of the switching circuit in 1.5× mode.In 1.5× mode, switching circuit 12 switches between the configurationsdenoted as phase A and phase B to generate an output voltage V_(OUT)that is 1.5 times the input voltage V_(IN). In phase A configuration,capacitor C1 is connected between V_(IN) and ground, while capacitor C2is connected between V_(IN) and V_(OUT). In phase B configuration,capacitors C1 and C2 are connected in series with opposite polaritybetween V_(IN) and V_(OUT). That the gain is 1.5 may be shownmathematically as follows, assuming no load on the output, that thecapacitors have charged/discharged to a steady state, and no voltagedrop across the switches. For the purpose of the following equations,V_(C1) and V_(C2) are the voltages across capacitors C1 and C2,respectively. By inspection of phase A of FIG. 3A, it is apparent thatthe voltages across the capacitors C1 and C2 are:

V_(C1)=V_(IN)  (1)

V_(C2)=V_(OUT)−V_(IN)  (2)

[0021] When the switches of switching circuit 12 are configured so as toproduce the circuit relationship of Phase B, the following relationshipis observed (particularly noting that C1 and C2 are connected in seriesin opposing polarity, i.e., the negatively charged side of eachcapacitor shares a node):

V_(OUT)−V_(IN)=V_(C1)−V_(C2)  (3)

[0022] Substituting from (1) and (2) into (3),

V_(OUT)−V_(IN)=V_(IN)−(V_(OUT)−V_(IN))  (4)

2V_(OUT)=3V_(IN)  (5)

[0023] $\begin{matrix}{V_{OUT} = {\frac{3}{2}V_{IN}}} & (6)\end{matrix}$

[0024]FIG. 3B depicts the phases of the switching circuit 12 in 2× mode.In 2× mode, switching circuit 12 switches between the configurationsalso denoted as phase A and phase B to generate an output voltageV_(OUT) that is 2 times the input voltage V_(IN). In phase Aconfiguration, capacitor C1 is connected between V_(IN) and ground,while capacitor C2 is connected between V_(IN) and V_(OUT). It should benoted that phase A in 2× mode is identical to phase A in 1.5× mode. Inphase B configuration, capacitor C2 is connected between V_(IN) andground, while capacitor C1 is connected between V_(IN) and V_(OUT). Thatthe circuits of FIG. 3B generate a 2× gain is shown mathematically asfollows, using an analysis similar to that above. Indeed, for Phase A,the equations are the same:

V_(C1)=V_(IN)  (7)

V_(C2)=V_(OUT)−V_(IN)  (8)

[0025] Considering Phase B, the opposite is true:

V_(C1)=V_(OUT)−V_(IN)  (9)

VC_(C2 =V) _(IN)  (10)

[0026] Substituting from (8) into (10)

V_(OUT)−V_(IN)=V_(IN)  (11)

V_(OUT)=2V_(IN)  (12)

[0027] Although switched mode charge pumps are referred to in the art asDC-DC power supplies, the output is not strictly DC. Due to theswitching between phases and consequent charging and discharging of thecharge storage capacitors C1 and C2, an AC component, called the“ripple,” is superimposed on the DC output. The ripple is an inherentcharacteristic of switch mode power supplies. Most charge pumps known inthe art strictly separate the two or more phases into charge anddischarge phases. That is, all of the charge storage capacitors arecharged from the input voltage during the first phase of operation andthen discharged to the output during the second phase. Thus, there is nocurrent supplied to the output during the charging phase. This mode ofoperation exacerbates the ripple component of the output. In contrast,in the charge pump modes depicted in FIG. 3A and 3B, current iscontinuously supplied to the output during both phases. This not onlyreduces the ripple, but also allows the charge pump to be operated insuch modes without the output capacitor 22 connected.

[0028] As used herein, a “continuous” supply of current denotes thatcurrent flows from a charge storage capacitor to the output during eachactive or operative phase. As will be appreciated by those of ordinaryskill in the art, in switching between the phases, i.e., switching theconfiguration of capacitors relative to the input and output nodes,avoiding overlap of the switch actuation is important to prevent chargeleakage or short circuit conditions. Hence, in a given application, theswitching controls may be designed to effect “break before make” switchactuation, i.e., switches that isolate nodes are opened prior to theswitches connecting nodes being closed. This may result in brieftransients while switching between phases when current is not suppliedto the output. As used herein, such current would still be considered“continuous” if it is supplied to the output during all phases of thecharge pump.

[0029]FIG. 4 is a functional diagram of the control circuit 14. Thecontrol circuit 14 controls actuation of the switches S1-S10 in theswitching circuit 12. The control circuit 14 comprises a mode selectioncircuit 30, a phase generator 32, a voltage reference 34, a clock 36, aswitch control circuit 38, and a switch buffer circuit 40.

[0030] The mode selection circuit 30 determines the operating mode ofthe charge pump 10, based on comparing the supply voltage, V_(BATT),with a reference voltage provided by the voltage reference 34. As willbe explained in more detail later, overall efficiency of the charge pump10 increases based on the mode selection circuit 30 choosing anoperating mode appropriate to the actual supply voltage and load. Themode selection circuit 30 provides a mode selection signal to the phasegenerator 32, with both the phase generator 32 and the mode selectioncircuit 30 being clocked synchronously by the clock 36. The clock 36provides a periodic clock signal CLK and a delayed version of the CLKsignal, referred to as CLKD. Both CLK and CLKD are used by various logiccircuits comprising the control circuit 14.

[0031] The phase generator 32 provides the switch control circuit 38with mode and phase signals that the switch control circuit 38 uses tocontrol which switches are actuated in the switching circuit 12, and inwhat sequence those switches are actuated. Thus, the phase generator 32implements the switching sequences in the switching circuit 12 thatdefine the mode of operation selected by the mode selection circuit 30.The switch buffer circuit 40 provides the on/off switching signalsrequired to actually drive the switches within the switching circuit 12in response to the control signals it receives from the switchingcontrol circuit 38.

[0032] As alluded to above, the need for a plurality of modes, providingdifferent gains or boost factors of the output voltage to input voltage,arises in a quest for efficiency that minimizes power dissipation andmaximizes battery life. The batteries that power portable electronicsdevices, such as for example, nickel cadmium, lithium ion, ornon-rechargeable alkaline batteries, lose their charge with continueduse. Typically, the battery voltage V_(BATT) gradually decreases overthe battery's useful life or charge cycle, and then rapidly loses itsremaining charge until little or no voltage is supplied. At this point,the battery must be recharged or replaced. In order to provide aregulated, constant output voltage V_(OUT) to the portable deviceelectronics, a charge pump typically boosts the battery voltage to alevel higher than that required by the output, and subsequentlyregulates the voltage down to the required level by use of a linearvoltage regulator at the output.

[0033] According to the present invention, the charge pump 10 produces aregulated output voltage V_(OUT) at the desired level by regulating thebattery voltage V_(BATT) at its input to the charge pump 10, andboosting this predetermined voltage by a known gain factor to producethe desired output voltage. This approach yields numerous advantagesover prior art output regulated charge pumps, as will be furtherexplicated herein.

[0034] If the charge pump 10 were to be operated in a single mode, toprovide power as the battery voltage decreases throughout its usefullife, a relatively large gain would be required, with the relativelyhigh voltage output of a fresh or fully charged battery being severelyregulated down to a low level. This would be very inefficient. Rather,to provide efficient power regulation throughout the range of voltagesgenerated by the battery, it is known to utilize a multi-phase approach,employing charge pumps of greater gain as required as the batteryvoltage V_(BATT) decreases.

[0035]FIG. 5 shows a graph of power efficiency of the charge pump 10over a subset of a typical battery's output voltage. The battery voltagedecreases from right to left along the abscissa axis of this graph asthe battery's charge is used during operation of the electronic device.The ordinate axis of the graph depicts the efficiency of the charge pump10. As depicted in FIG. 5, in the region at the right of the graph, thecharge pump 10 is in a 1.5× gain mode (the configuration depicted inFIG. 3A). To provide, in the example depicted in FIG. 5, a nominal 5.5 Voutput with a 1.5× gain, the input to switching circuit 12 should beapproximately 3.7 V. At battery voltages higher than 3.7 volts, avoltage regulator within the input circuit 16 regulates the batteryvoltage down to the required approximately 3.7 volts. When the batteryvoltage greatly exceeds 3.7 V, the efficiency of charge pump 10 suffers,as power is lost due to the input regulation.

[0036] As the battery voltage decays to within the range of 3.7 volts,the charge pump 10 achieves its maximum efficiency, as the voltageregulator of the input circuit 16 passes the 3.7 V directly to theswitching circuit 12 to be boosted to the output. However, as thebattery voltage continues to drop below 3.7 V, a boost of 1.5× cannotproduce the required output of 5.5 V. Thus, a stronger gain mode ofcharge pump 10 is required, and the control circuit 14 will actuateswitches within the switching circuit 12, configuring the charge storagecapacitors C1 and C2 into the 2× mode (as depicted in FIG. 3B). At thispoint, the relatively high voltage, e.g., 3.6 V, when doubled, wouldexceed the nominal required output of 5.5 V.

[0037] The input voltage is thus regulated down by input circuit 16 toapproximately 2.8 V. The drop in the graph of FIG. 5 to the left ofV_(BATT)=3.7 V shows the inefficiency due to the power lost in thisvoltage regulation. As the battery voltage continues to drop, theefficiency of charge pump 10 gradually increases as the requiredregulation decreases, reaching its maximum efficiency as the batteryvoltage approaches 2.8 V. Similarly, as the battery voltage decays below2.8 volts, the switching circuit 12 is configured to a 3× gain mode (notshown), and the available battery voltage is regulated down toapproximately 1.8 V by the input circuit 16. In this manner, amulti-mode charge pump 10 can provide a regulated output voltage V_(OUT)over a wide range of battery voltages V_(BATT) while maintaining arelatively high efficiency.

[0038] During the changes between modes of the multimode switchingcircuit 12, various undesirable effects may be manifest. For example,when switching from the 1.5× to the 2× mode, large transient currentsmay be induced through the V_(IN) node to the switching circuit 12.Considering FIG. 3A, at the end of phase A, charge storage capacitor C1will be charged to V_(IN), and capacitor C2 will be at ½ V_(IN). Duringsteady state conditions, the same charge is across the capacitors at theend of phase B. Upon switching to the 2× mode, as depicted in FIG. 3B,the phase A configuration is the same as the phase A configuration ofthe 1.5× mode, i.e., capacitor C1 at V_(IN) and capacitor C2 and ½V_(IN). Upon switching to phase B of FIG. 3B however, capacitor C2, witha charge of ½ V_(IN), will attempt to rapidly charge to V_(IN),resulting in a large current flow through the input node to capacitorC2.

[0039] This undesired large transient current may pull down the outputvoltage of the battery V_(BATT), induce noise into the system, anddissipate excessive power. According to the present invention, and as ismore fully explicated herein, the input circuit 16 of charge pump 10 mayinclude current limiting capability, thus sensing excessive currents atthe V_(IN) node and limiting current from the battery to an acceptablelevel. This allows for a smoother transition between gain modes, as thecharge storage capacitors C1 and C2 charge to their new values over thecourse of several phase changes.

[0040] The output voltage V_(BATT) of the battery, while generallydecreasing over time, does not do so in a strictly linear or evenpredictable manner. Environmental factors such as ambient temperatureand humidity, transients in the load, and the operation of batteryrecharging circuits may all contribute to an increase in the output ofbattery voltage V_(BATT). If this increase in battery voltage occursnear a mode switching point, the charge pump 10 may need to switch froma higher gain mode to a lower gain mode. Referring again to FIGS. 3A and3B, consider the case where the charge pump 10 switches from the 2× modeof FIG. 3B to the 1.5× mode of FIG. 3A. In a steady state condition andwith no load applied, the node at the positive side of the storagecapacitor C2 is charged to V_(IN), and the node at the positive side ofstorage capacitor C1 is charged to 2V_(IN). Upon switching to phase A ofthe 1.5× mode as depicted in FIG. 3A, V_(OUT) remains at 2V_(IN), asthis is the same configuration as phase A of the 2× mode. Upon switchingto phase B of the 1.5× mode, however, capacitor C2 places a2V_(IN)charge on the input node that is at voltage level V_(IN). Thisreverse biases the input node, and without protection will cause currentto flow from the charge pump 10 into the battery 26, potentiallydamaging the battery and pulling down the output voltage of the chargepump 10. To prevent this condition, the voltage regulator in inputcircuit 16 may additionally serve as an isolation switch to remove theV_(IN) node of switching circuit 12 from the output of the battery 26.The input circuit 16 also outputs a reverse bias detection signal sothat the control circuit 14 may additionally open switches within theswitching circuit 12 to further isolate the charge storage capacitorsfrom the input.

[0041] The input circuit 16 is depicted in FIG. 6. The input circuit 16comprises a voltage regulator 50, a differential voltage sensor 60, acurrent limit circuit 62 and its associated reference transistor 64, anda reverse bias protection circuit 66. In this exemplary embodiment, theinput circuit 16 is disposed between the output of the battery 26 andthe input node of the switching circuit 12. However, at least some ofthe various functions of the input circuit, such as its current limitingfunction, may be distributed elsewhere within the charge pump 10.

[0042] Voltage regulator 50 compares the output voltage V_(OUT) of theswitching circuit 12 to a reference voltage V_(REF). The regulator 50regulates the battery voltage V_(BATT) such that the input voltageV_(IN) provided to the switching circuit results in the desired outputvoltage V_(OUT) The voltage regulator 50 comprises an amplifier 52, apass transistor 54, and resistors 56 and 58. The resistors 56 and 58form a voltage divider that provides a fraction of the V_(OUT) voltageto one input of the amplifier 52. The amplifier 52 generates an errorsignal based on a difference between a reference voltage V_(REF) appliedto the other input of the amplifier and the output from the voltagedivider.

[0043] By driving the gate of the pass transistor with this errorsignal, the pass transistor 54 is maintained at a gate bias thatmaintains V_(IN) at a voltage resulting in the desired value of V_(OUT).The output of the charge pump 10 is thus continuously and automaticallyregulated to the desired output level V_(OUT) by operation of voltageregulator 50.

[0044] Regulating the input voltage V_(IN) to the switching circuit 12presents several advantages over the prior art practice of regulatingthe output voltage V_(OUT). By regulating the input voltage,proportionally smaller voltages need to be regulated down than at theoutput of charge pump 10, particularly for large gain modes of theswitching circuit 12. This results in the voltage regulator 50 operatingwith lower voltage differentials between V_(BATT) and V_(IN).Additionally, increasing integration, decreasing size, and reduced powerconsumption of modern integrated circuits results in smaller featuresize, i.e., reduced size of the transistors and other components formedin the integrated circuit die. These smaller geometries, although fasterand dissipating less power, are more susceptible to damage from highvoltages. Specifically, the switches comprising the switching circuit 12may be formed from Field Effect Transistors (FETs) or similar structureson an integrated circuit. FETs can only withstand certain maximumvoltages, dependent on feature size and process technology, across theirswitching terminals, i.e., from the drain to source nodes, denotedV_(DS). Voltages exceeding a maximum permissible V_(DS) may permanentlydamage the integrated circuit, rendering the charge pump 10 inoperative.Thus, regulating the battery voltage V_(BATT) down to known valuesbefore applying it to the input node of the switching circuit 12 ensuresthat excessive V_(DS) voltages do not appear across the FET switches.This is of particular concern when switching from one gain mode to arelatively higher gain mode, when the battery voltage V_(BATT) is at itshighest value for the higher gain mode range.

[0045] The pass transistor 54 can also serve as an isolation switch,isolating the battery 26 from the switching circuit 12 during reversebias conditions that can arise as the charge pump 10 changes operatingmodes. The reverse bias condition arises when one of charge storagecapacitors associated with the switching circuit 12 is connected to theV_(IN) input node of the switching circuit 12 when it charged to avoltage level higher than V_(BATT). This type of condition might existmomentarily, for example, when changing from a 2× mode to a 1.5× mode.FET devices, with their low on-resistance, are ideal for use as the passtransistor 54. However, practical FET devices often include a parasiticdiode between their drain and source that would allow current to flowfrom the V_(IN) connection back to the V_(BATT) connection wheneverV_(IN) exceeded V_(BATT). Thus, the V_(DS) sense circuit 60 senses thedrain-source voltage across the pass transistor 54 to detect suchreverse bias conditions.

[0046] The V_(DS) sense circuit, which may be implemented as adifferential amplifier, provides a control signal to the reverse biasprotection circuit 66. The reverse bias protection circuit 66 turns offthe pass transistor 54, and asserts a control signal to the controlcircuit 14, causing the control circuit 14 to control the switchingcircuit 12 to disconnect its associated charge storage capacitors 18 or20 from the V_(IN) input node. Alternatively, the control signal may beasserted directly to the switching circuit 12. Controlling the switchingcircuit 12 in this manner avoids having one of the capacitors 18 and 20from discharging into the battery 26 back through the parasitic diode ofthe pass transistor 54.

[0047] Reverse current may also be limited when transitioning from arelatively stronger gain mode to a relatively weaker one by the controlcircuit 14 forcing the switching circuit 12 to remain in the “charge”phase (i.e., phase A as depicted in FIGS. 3A and 3B). This maintainsconstant current flow to the output, and additionally assists transitionto the weaker gain mode by charging the charge storage capacitors asrequired for the new mode. The switching circuit 12 may be forced toremain in charge mode by the control circuit 14 immediately uponchanging modes, based on knowledge of the mode change. Additionally, thecontrol circuit 14 may force the switching circuit 12 into the chargephase for a given mode whenever the reverse bias condition is detected.

[0048] The pass transistor 54 additionally may limit the current passingthrough the V_(IN) node to switching circuit 12. Current limiting may beindicated, for example, when large load transients attempt to drawexcessive current from the charge pump 10, or to limit transientcurrents when switching modes, as described above. The referencetransistor 64 is configured with a known geometry relative to the passtransistor 54 and controlled such that it has the same drain-to-sourcedifferential voltage as the pass transistor 54. The current limitcircuit 62 effects this drain-to-source voltage control responsive tothe control signal provided by the V_(DS) sense circuit 60. With thesame drain-to-source voltage and same gate bias as the pass transistor54, the current through the reference transistor 64 has a knownrelationship to the current through the pass transistor 54.

[0049] Sensing when the current through the reference transistor exceedsa given threshold allows the current limit circuit 62 to effectivelysense when the current through the pass transistor 54 exceeds aproportional threshold. Thus, the current limit circuit 62 may be set tolimit the current through the pass transistor 54 to a desired maximumvalue. When the current through the pass transistor 54 approaches thismaximum value, the current limit circuit 62 controls the gate of thepass transistor to limit current to the maximum value.

[0050] Additionally, when transitioning from a relatively weaker gainmode to a relatively stronger gain mode, the control circuit 14 maydecrease the response time of the current limiting function of the inputcircuit 16 by precharging the current limiting circuit 62. The output ofthe current limiting circuit 62 sources current to the gate of the passtransistor 54, to limit current through transistor 54. Based on thepending mode transition, the control circuit 14 may predict largetransient currents, and force the current limiting circuit 62 to biasthe drain of transistor 54 into current limiting mode in advance of themode change. Thereafter, the current limiting circuit 62 monitorscurrent through the reference transistor 64 and controls thecorresponding current through transistor 54, as described above.

[0051] Many other techniques for controlling current in the charge pump10 may occur to one of ordinary skill in the art. For example, currentmay be limited by restricting the on-time duration of one or moreswitches in the switching circuit 12 during each phase. Alternatively,particularly for implementations utilizing MosFET switches, the gate tosource voltage of one or more switches may be controlled, such that theswitch is not fully “on.” Additionally or alternatively, the slew rateson the switching transitions may be lengthened.

[0052] Thus, while the present invention has been described herein withrespect to particular features, aspects and embodiments thereof, it willbe apparent that numerous variations, modifications, and otherembodiments are possible within the broad scope of the presentinvention, and accordingly, all variations, modifications andembodiments are to be regarded as being within the spirit and scope ofthe invention. The present embodiments are therefore to be construed inall aspects as illustrative and not restrictive and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein.

What is claimed is:
 1. A multiphase charge pump power supply, comprisinga) an input connected to a voltage source; b) an output providing aregulated output voltage; c) at least two capacitors, including a firstand a second capacitor; d) a switching circuit providing a continuouscurrent at said output comprising a plurality of switches and said atleast two capacitors; said switching circuit alternating between a firstphase and a second phase by selective actuation of said switches; e)wherein in said first phase said first capacitor is connected betweensaid input and said output and said second capacitor is connectedbetween said input and said ground; and f) wherein in a second phasesaid first and second capacitors are connected in series between saidinput and said output.
 2. The multiphase charge power supply of claim 1,wherein in said second phase, said first and second capacitors areconnected in series with opposite polarity.
 3. The multiphase chargepower supply of claim 1 further comprising an input protection circuitconnected between said input and said voltage source.
 4. The multiphasecharge pump power supply of claim 4 wherein said input protectioncircuit is a current limiting circuit to limit current flow from saidvoltage source.
 5. The multiphase charge pump power supply of claim 3wherein said input protection circuit controls a switch within saidswitching circuit to prevent current reversal through said input.
 6. Themultiphase charge pump power supply of claim 3 wherein said inputprotection circuit causes said switching circuit to switch to said firstphase when a negative current through said input is sensed.
 7. Themultiphase charge pump power supply of claim 4 wherein said inputprotection circuit comprises a linear voltage regulator to limit theinput voltage at said input of said switching circuit and to preventbreakdown of said switches in said switching circuit.
 8. A multiphasecharge pump power supply, comprising: a) an input connected to voltagesource; b) an output providing a regulated output voltage; and c) aswitching circuit comprising a plurality of switches and at least twocapacitors; said switching circuit alternating between a first phase anda second phase by selective actuation of said switches to provide saidregulated output voltage at said output; and d) an input protectioncircuit connected between said input and said voltage source.
 9. Themultiphase charge pump power supply of claim 8 wherein said inputprotection circuit is a current limiting circuit to limit current flowfrom said voltage source.
 10. The multiphase charge pump power supply ofclaim 8 wherein said input protection circuit controls a switch withinsaid switching circuit to prevent current reversal through said input.11. The multiphase charge pump power supply of claim 8 wherein saidinput protection circuit comprises a linear voltage regulator to preventvoltage to limit the input voltage at said input of said switchingcircuit and to prevent breakdown of said switches in said switchingcircuit.
 12. The multiphase charge pump power supply of claim 8 whereinsaid input protection circuit causes said switching circuit to switch tosaid first phase when a negative current through said input is sensed.13. A multiphase charge pump power supply with reverse bias inputisolation, comprising: a) an input connected to a voltage source; b) anoutput providing a regulated output voltage; c) a switching circuitconnected between said input and said output comprising a plurality ofswitches and said at least two capacitors; said switching circuitalternating between a first phase and a second phase by selectiveactuation of said switches to produce said regulated output voltage; andd) an input protection circuit connected between said input voltagesource and said input to detect a reverse bias condition where saidinput voltage is greater than said input voltage source voltage and tocontrol at least one switch in said switching circuit to prevent currentreversal through said input.
 14. The multiphase charge pump power supplyof claim 13 wherein said input protection circuit causes said switchingcircuit to switch to said first phase when a negative current throughsaid input is sensed.
 15. The multiphase charge pump power supply ofclaim 13 wherein input protection circuit further comprises a currentlimiting circuit to limit current flowing from said voltage source. 16.The multiphase charge pump power supply of claim 13 wherein inputprotection circuit further comprises wherein said input protectioncircuit further comprises a linear voltage regulator to limit the inputvoltage at said input of said switching circuit and to prevent breakdownof said switches in said switching circuit.
 17. A method of providing agenerally constant output voltage to a load from a voltage sourceproducing a variable input voltage, comprising: a) selectively actuatingswitches within a switching circuit to toggle said switching circuitbetween at least two distinct phases to provide a regulated outputvoltage at an output of said power supply; b) when a reverse biascondition is detected, electrically isolating said switching circuitfrom said voltage source via selective actuation of at least one switchin said switching circuit.
 18. The method of claim 17, furthercomprising restricting said switching circuit to one of said at leasttwo distinct phases to prevent current reversal into said voltage sourcewhen said reverse bias condition is sensed
 19. The method of claim 17,further comprising limiting the current flowing from said voltage sourceto said switching circuit to a predetermined value.
 20. The method ofclaim 17, further comprising regulating the voltage at said input ofsaid switching circuit.
 21. A method of providing a generally constantoutput voltage to a load from a voltage source producing a variableinput voltage, comprising: a) selectively actuating switches within aswitching circuit to toggle said switching circuit between at least twodistinct phases to provide a regulated output voltage at an output ofsaid power supply; and b) limiting the current flowing from said voltagesource to said switching circuit to a predetermined value.
 22. Themethod of claim 21, further comprising restricting said switchingcircuit to one of said at least two distinct phases to prevent currentreversal into said voltage source.
 23. The method of claim 21 furthercomprising regulating the voltage at said input of said switchingcircuit.
 24. A method of providing a generally constant output voltageto a load from a voltage source producing a variable input voltage,comprising: a) selectively actuating switches within a switching circuitto toggle said switching circuit between at least two distinct phases toprovide a regulated output voltage at an output of said power supply;and b) regulating the voltage at said input of said switching circuit.25. A multimode, multiphase charge pump power supply, comprising a) aninput connected to a voltage source; b) an output providing a regulatedoutput voltage; d) a switching circuit connected between said input andsaid output, said switching circuit comprising a plurality of switchesoperable in at least two different operating modes, each operating modecomprising a plurality of phases; e) a control circuit to select theoperating mode of said switching circuit and to generate a switchingsignal to alternate between said plurality of phases of the selectedoperating mode during a switching cycle; and f) said switching circuitbeing further operative to suppress said switching signal for at leastone switching cycle following a change from a first operating mode to asecond operating mode.